Archaeological Geophysics with Snuffler

14 August 2017

I was recently asked by the owners of Quarry Farm, Bodiam, to look at the Roman roadside settlement on their land, which is right up my (Roman) street. They were kind enough to put me up in their fantastic glamping site, with views across the settlement to the castle :

View to the castle from the glamping hut

The site was excavated back in the 1960's, when the Battle and District Historical Society dug up a Roman building with Classis Britannica stamped tiles associated with it. The Roman road broadly follows the modern road across the floodplain, and the presence of the CL:BR stamped tiles in the floodplain has led to the interpretation of the site as a port. Further to the south, as the land rises out of the floodplain, some iron slag has been found, suggesting an iron working site.

Lidar shows up an interesting feature in the floodplain, an embanked paleochannel that cuts much further south than the current course of the river. It has been suggested that the river was redirected further north in medieval times to help fill the moat. Also look at the two fields just south of the floodplain, and notice that the field on the left is significantly higher than the field on the right.

lidar of the site, click for larger image

So what about the geophysics? Starting with magnetometry, the reason for the height difference between the two fields is obvious, as there is a medium sized iron working site, whose slag heap has raised the level of that field compared to the one next door. You can also see a slag metalled track leading from the Roman road (to the east) to the iron working site. The floodplain is not so easy to interpret. There is a lot of metal junk, geological features, and material from the railway and modern road. It's difficult to say with certainty from the magnetometry that anything in there is Roman, despite the amount of Roman material found in excavation.

Magnetometry, click for larger image

It's down to the radar to help sort the mess out. The orange lines mark out the areas surveyed with GPR. The black rectangle is the Battle and District Historical Society excavation area, in which was found the building they excavated. That building doesn't show up on the radar, which is not surprising, as the excavations were described as waterlogged, and GPR doesn't do well with wet alluvium. There is what looks like new building just next to it though, closer to the road. Most interesting though is an oval bowl shaped feature surround on three sides by hard standing. My gut says this is an upstream port, but the paleochannel seems to go through it rather than by the side of it, so I'm not sure what to make of it.

Interpretation of floodplain GPR, click for larger image

Back at the iron-working site, the radar showed the extent of the slag heap as an amplitude change on the enveloped data, and also showed the location of three bloomeries as circular phase change features on the unenveloped data.

Interpretation of iron working GPR, click for larger image

So there's a lot going on, but not quite as large a settlement as expected. If you want to know more, you can read the full report here.

If you look at the above image, the test pit from last year is on the western end of the building marked 'B', targetting the surviving floors, while the larger trench from this year targets the smaller building marked 'C'.

Click for larger image

What is left of the floor fills most of the test pit in the above image, unfortunately with no surviving mosaics. There are walls to the west and north, with a cut representing a robber trench for another wall to the east, over which can be seen part of the remaining floor in the next room

Click for larger image

The above image shows the southern end of the larger trench from this year, looking west. The funny shape of the end of the wall visible in the radar is also visible in plan. The stacks of tile are pilae forming part of a hypocaust heating system, over which would have been an opus floor, part of which is visible sticking out of the baulk at the bottom of the image. That floor, and the pilae, don't appear on the radar data though, so clay products don't seem to have a difference in contrast to the local soil. The building is most likely a small bath house, which despite being built next to the building to the north, is actually attached to the building to the south. This distance is so that if the building goes up in flames because of the under floor heating, the rest of the building is not threatened. Dating seems to suggest a date for the building in the last Roman period.

Congratulations to Chichester on a fantastic dig, and watch this space for more results if they dig more next year.

06 April 2017

Erica Utsi, whose name is on my GPR and is shortly to publish a book recently became a TV star after appearing on a program about William Shakespeare's grave, and given we were a specialist audience rather than the TV viewing public, we got a slightly more in depth explanation of what it was all about, which was that Shakespeare's head may have been nicked due to a fashion for collecting the skulls of famous people.

Adam Booth treated us to some technology not often linked to the sort of geophysics we do, portable x-ray fluorescence, which can be used to identify the elements present in a sample, without having to a lab. His test site was the site of a WWII plane crash. Parts of the plane were visible on magnetometry, so what did the new tech turn up to go with that? In a transect across the site, a spike in copper and zinc from the remains of the plane leaching into the surrounding soil was visible. I've seen a similar talk where the technology was used in industrial sites, where it was suggested that it was useful for identifying prehistoric metal working, which may have been little more than a campfire affair. Always nice to see new tech explained.

My favourite subject, Roman roads, got a mention by Joep Orbons, who had thrown quite a few geophysics techniques (EM, Mag, ER, ERT and GPR) at a section of Roman road in Belgium. The sort of results he got were very familiar to me, with differences in preservation and different soil conditions giving different results of quite a small area, with some techniques (GPR, ER) performing better than others. Sometimes even massive features like this can be hard to find. Not content with one talk on Roman roads, that last talk was immediately followed by Michal Pisz taking about the Roman fort of Tibiscum in Romania with the Roman roads and surrounding vicus being surveyed and excavated.

Already mentioned in this blog, Chris Lockyear has been producing some amazing results on the Roman town of Verulanium here in the UK, with multiple buildings, roads and an aqueduct visible in surveys carried out using ER, Mag and GPR. The preservation is fantastic, and I really hope they find a lot more like it. If you haven't seen it already, check out their blog. Talking of big, pretty surveys, Tomasz Herbich spoke after Chris and has been researching ancient towns in Egypt, with predictably decent results from magnetometry due to the use of fired brick.

H Webber suggested a new avenue of research for archaeologists, using the vast geophysical surveys, such as EM, carried out for the benefit of farmers in modern day precision agriculture. Phosphates present in occupation material may highlight areas of occupation that the archaeological community were not previously aware of. Of course, the farmers would have to be approached in order to get this data, and someone in the audience pointed out that if all of this was explained to the farmers, some of them might deep plough the sites away in order to bring the free fertiliser to the surface.

Petra Schneidhofer gave us a talk about the state of geophysics in Norway and Denmark. Apparently, igneous geologies make our usual favourite, magnetometry, rather pointless, so GPR is commonly used instead. despite that, the natural variation over an area in GPR is quite extreme, and it can be quite difficult to pick out features. Thanks goodness for the boring sedimentary geologies in my part of the world.

It's that time of year when the weather is getting a bit warmer and it is time for me to wander once more into the green fields of England, with a machine that goes beep, to find the lost wossnames of times past. It isn't just my own Roman period projects that I work on though, I also do work for various local societies, as many don't have their own geophysics equipment. Here's a selection of projects that I've been involved with recently.

The Pepperpot, Brighton

At the end of Tower Road, Brighton, there is a tower (no surprise there) which apparently used to house pumping equipment for a well that supplied the Attree Villa and estate. There was apparently a water tank and an underground tunnel under what is now the road, and Brighton and Hove Archaeological Society along with the Friends of the Pepperpot asked me to take a look with my radar. There were signs of rubble in the area where the water tank would have been, and very vague signs of the tunnel, but the results weren't all that clear. You can see the full report here.

The interpretation of the GPR survey over an old map of the area around the Pepperpot

Butts Brow Neolithic Enclosure, Eastbourne

Though mostly filled with a combination of car park and a clump of trees, there is a second neolithic enclosure above Willingdon, Eastbourne (the first being the more well known Combe Hill causewayed enclosure). After a season of excavation targetting the surviving sections of bank and ditch by the Eastbourne Natural History and Archaeology Society, I was asked to see if I could find them some internal features to dig up. It's rather difficult to see cuts in chalk with radar, especially with modern tracks and bands of natural flint around, but the ditch was slightly visible as a negative feature cutting through the flint layers. It's the dark band in the image below. The contrast between the ditch and surrounding chalk was very slight though, so smaller internal features were not visible. You can see the full report here and you can see a video of the results here. Details of a dig this summer will be published here at some point.

The neolithic ditch cutting through a band of natural flint

Southborough Post Mill

Just over the border into Kent this time, the Southborough and High Brooms Amateur Archaeological Society asked me to look at a platform in the woods of Southborough Common, the site of a post mill. Geophysics surveys in woodlands are never easy, and while the woods had been cleared, some trees remained. Both earth resistance and magnetometry were used, the results of which are in the channel merge image below. The magnetometry didn't show much apart from a big chunk of metal and some surrounding (no longer visible) fencing, the earth resistance showed a high resistance area on the east side of the platform, which may have been the site of the mill. You can see the full report here.

29 December 2016

The twin-probe earth
resistance meter, being relatively cheap, is often the first piece of
geophysics equipment purchased by local archaeological societies.
While it may not be the first port of call if you have access to a
magnetometer or GPR, there are many situations where it is superior.
I've found that earth resistance is the most reliable method for
finding Roman roads. Recently, I've had access to multiple pieces of
equipment, so I have decided to do a review.

The first of the three
machines is the Geoscan RM15. Now replaced by the RM85, which
unfortunately I don't have access to, the only major differences that
I'm aware of is the inclusion of the multiplexer within the box
rather than as an add-on, GPS logging and output via USB instead of
the old serial port. If there are further changes that would change
this review, I apologise to Geoscan now.

The second machine is the TR
Systems meter, which was aimed at local societies and proved very
popular before production ceased. Though it is not available any
more, its use is so widespread that I include it here for comparison
purposes, as many will be familiar with it.

The third machine is the
Frobisher TAR-3, a relative newcomer, and like the TR Systems meter,
affordable by local societies on a budget.

User Interface

The best way to introduce
this section is with images of the interfaces of each machine.

Geoscan
RM15 Interface

TR Systems
Interface

Frobisher
TAR-3 Interface

Both the RM15 and TR
machines have a similar interface style, with buttons for each
function. The TR machine seems to have taken a design lead from the
Geoscan machine, no doubt hoping that familiarity will translate into
ease of use. The Frobisher machine has a more minimalist style, with
5 buttons (duplicated, for left handers) controlling a menu system,
similar to that used by Bartington in their GRAD601. Ease of use is
subjective, and somewhat reliant on familiarity, but some comments
can be made.

The Geoscan machine is
probably the easiest to use. The TR Systems meter works in much the
same way, but has an annoying feature where instead of beeping once
when a reading is taken, it will beep when it is starting to take the
reading and beep a second time when it is finished. If you take the
probes out too early, before the second beep, it will complain
furiously, saying something about checking the probes, when you know
it is because you took the probes out too early, and you have to wait
several seconds before it will allow you to continue. I gather that
this 'feature' is due to listening to feedback from users who really
should not have been listened to. The Frobisher, lacking the
dedicated buttons for each function, is probably the least intuitive,
and you will probably need the manual at hand the first few times you
use it, until you get used to it. Training is available though. There
are inconsistencies with the beeps to record a reading, so at the end
of line beep, there is a pause and a further beep which may
incorrectly suggest that another reading hasn't been taken, and when
you are retaking a reading, there is no beep to say it has been
taken. The other strange design decision relates to the end of the
grid. It will take 20 seconds to write out the readings to its
storage, and then turn itself off, cancelling out the speed increase
afforded by the ergonomic design. Hopefully some of these issues will
be resolved with firmware updates.

Verdict: 1st –
Geoscan, 2nd – TR Systems, 3rd - Frobisher

Ergonomics

A big part
of the 'experience' of doing an earth resistance survey is lugging
the machine around the survey area, over and over again, so how your
equipment handles is of great importance. A common criticism of
equipment like this is the effect it has on someone with a bad back,
both because of the weight of the equipment, and because the height
of the bar which you hold on to can make you stoop somewhat. With
that in mind, here is a table with some statistics on the three
machines.

Machine

Weight (sans cables)

Bar Height

Geoscan RM15

5.1Kg

93cm

TR Systems

4.4Kg

93cm

Frobisher TAR-3

2.8Kg

105cm

As you can
see, the Frobisher is much lighter and has a higher bar than the
other two. My volunteer, Stuart, who has a history of back problems,
reported that the Frobisher was his favourite. Another beneficial
side effect of a ligher machine is the ability to move it quicker,
meaning the survey area is covered quicker. Frobisher can supply
whatever bar height required on ordering, including a childrens size
frame (40cm-130cm).

Verdict:
1st - Frobisher, 2nd – TR Systems, 3rd – Geoscan

Hardware
Options

The biggest
selling point of the Geoscan RM85 has a built-in multiplexer, which
used to be a separate add-on to the RM15, so parallel and deeper
readings can be taken at the same time using the adjustable probe
frame (an additional option). The RM85 also has an option of GPS
recording if you are into using point clouds.

The TR
systems meter had an optional tomography kit for doing manual ERT
surveys and producing pseudosections using the free version of
RES3DINV.

The
Frobisher machine, being new, has yet to accumulate the same level of
hardware options as the other machines, but one very useful feature
is that the fixed probe cable is easily extendable, meaning more
grids can be surveyed without moving the fixed probes. The
manufacturer has mentioned that the cable could potentially be done
away with entirely, with an entirely separate transmitter, which
means very large areas could be done without moving the fixed probes,
so faster surveys and no edge matching in software required. A wenner
bar is available, and a tomography kit is in production.

Verdict:
It really depends what you find useful!

Battery

While I
can't compare battery life for each machine, I can comment on how
easy it is to change batteries.

The Geoscan
RM15 and RM85 have an internal battery pack of standard batteries
(normal or rechargable). The unit needs to be unscrewed to replace
the batteries, but it is possible to do this in the field.

The TR
Systems meter has two plastic trays that slot into the side of the
machine, so batteries (9V, standard or rechargable) can be easily
changed in the field.

The
Frobisher TAR-3 has an internal rechargable battery pack that is not
user accessible. If something goes wrong with the battery, the unit
must be returned to the manufacturer. It is charged via a USB
connector, so can be charged in the field using a car charger, or
anything that could charge a phone.

Verdict:
1st – TR Systems, 2nd – Geoscan, 3rd - Frobisher

Downloading
Data

The RM15 and
TR systems meter download via an old 9 pin serial connector, so you
would need a serial to USB converter or card to download the data.
Fortunately, the replacement for the RM15, the RM85, has now been
changed to a USB connector that mimics a serial port, no additional
hardware needed. The Frobisher TAR-3 stores data on an SD card that
can be read with any card reader, so getting the data onto your
computer is much faster.

Verdict:
1st – Frobisher, 2nd – Geoscan, 3rd – TR Systems

Data
Quality

The test
site was a park through which ran a Roman road. The park is
surrounded by buildings, which was an opportunity to see how the
three machines were affected by AC interference. The same fixed probe
location (0.5m apart) was used for each of the three surveys. The
area had been previously surveyed using GPR, and the road is visible
in the timeslices starting at about 30cm down, along with some land
drains or utilities. The surface is known to be made of flint, and
the local geology is on the boundary between Folkestone Formation
sandstone and Lower Greensand.

The GPR grid
shown above is 30x30m, and the earth resistance test grid occupies
the top-left 20x20m of that area. The results, shown below were
processed in Snuffler with no filters applied. The display bounds
were set to 95% of the readings around the median. There isn't much
evidence of noise on any of the three images, and they seem broadly
consistent with eachother.

Geoscan
RM15

TR Systems

Frobisher
TAR-3

Verdict: Not much to choose between them, make up your own
mind!

Price

When I
bought my TR systems meter, many years ago, the price was £1200.
Inflation would make that about £1800. At the time of writing, the
Frobisher TAR-3 is £1844 (including a days training), not very
different from the TR Systems meter, and aimed at the same budget
conscious market. I'm not absolutely sure of the price of the
currently Geoscan RM85, but I have been told the basic machine £5000,
with the multi-probe array another £1500.

Verdict:
Joint 1st – TR systems, Frobisher, 3rd – Geoscan

Conclusion

Given that
the TR Systems meter is not currently available, that leaves us with
the Geoscan and Frobisher machines. If you want the multiplexer
option, then get the RM85, otherwise the lower cost and lighter
Frobisher machine will save your back and bank balance.

29 October 2016

Following on from last year's very successful radar survey in Chichester, I went back for another week of the same, this time around the area of the Cathedral in the south-west quadrant of the city. Chichester & District Archaeological Society had already found a lot in the area using earth resistance and excavation, so the radar didn't show a lot that was new, just in slightly better definition. There wasn't a lot around the cathedral itself, so the area has probably had any Roman remains there thoroughly removed, but there was around the Deanery and Bishop's Palace. The garden of the Deanery contains the old medieval (or post-medieval) Deanery, and the the area in front of the Bishop's Palace contains a Roman building and the medieval hall that was the old Bishop's palace.

The old Deanery in the garden of the current Deanery

Click for larger image

Roman building (green) and medieval Bishop's Palace (orange)

Click for larger image

I also went back to Priory Park to look again at the third Roman building (in green, within the lighter survey area) found near the cricket pitch. It has suffered greatly from robbing, not least by the Saxons, who seem to have used some of the stone in their sunken floor buildings, two of which appear (in purple) in the higher resolution re-survey of this area.

Priory Park survey. Click for larger image.

I'll be giving a talk on the results from both years for CDAS on the 22nd of February 2017 at 7:30pm in the cinema of the New Park Centre, New
Park Road, Chichester.

04 October 2016

Since I found that my last blog post on displaying Environment Agency LIDAR DEMs has become the most ever viewed blog post that I have written (popular subject apparently), I've been thinking of writing a followup, having learned a few new things. One of the main problems with dealing with all this LIDAR data is speed. First, getting the data to a state that is useful takes a lot of processing, which can be solved by automating the process using some python scripts I wrote. Second, the draw speed on the screen in QGIS can be solved using Virtual Raster Tables and Pyramids. This tutorial will assume that you are using the OSGeo4W package on windows, with QGIS, python and OSGeo4W Shell options installed, but much of it may be transferable to other setups.

I'll start by throwing some python code at you for processing the data, and then explain a bit about what it does and how to run it. Put this code in a file somewhere called demimport.py

You will see near the top of this script two directories called impdir and expdir. impdir is the directory where you dump the zipfiles you download from the environment agency website. expdir is the directory where the script will output the resulting data. Change these to whatever directory structure you wish to use on your computer. The script will merge the contents of each zip file into a single file and then create two different hillshades from that data. More on the hillshades later. It automates most of what I described in my last blog post, all apart from setting the style data in QGIS. You can run the script by opening OSGeo4W Shell, changing directory to where you have saved the python file and typing @python demimport.py.

The next bit is about speeding up the data display in QGIS itself, using the aforementioned Virtual Raster Tables (hereafter, VRTs) and Pyramids. First, VRTs. Imagine you have 100 LIDAR tiles active for display in QGIS. Each time you zoom or move the display, it has to read them all to see which it can display in the area you are viewing at the time, which obviously means a lot of slow reading from your hard disk (assuming you still use those). A VRT acts as an index file for your 100 LIDAR tiles, so QGIS only has to look in one place to find out what it needs to display, and then only has to open the tiles that it requires to fill the area you are viewing, so everything displays much quicker. Pyramids speed things up in a different way. Imagine you are quite zoomed out, looking at a wide area of LIDAR data. QGIS would normally have to read the whole of each file and reduce it in size to fit into the small area that the tile would be displayed in. Creating a pyramid does this process ahead of time, so it takes the original image, compresses ito to a quarter of its size, then does that again and again and saves all that in a pyramid file, so when you are zoomed out, rather than reading the entirity of the original data, it will read the pre-compressed data suitable to your zoom level, reducing the amount it has to read from your hard disk and speeding up the display process. Those are the concepts behind it, now for the code that actually does it. Put this code in a file called makevrt.py

You can run the script by opening OSGeo4W Shell, changing directory to
where you have saved the python file and typing @python makevrt.py. Again, there are some changes you will need to make at the top of the file. resolution is the type of data you are dealing with. You can download 2M, 1M, 50CM or 25CM data from the Environment Agency website, this script will only do one at a time. expdir should be the same as expdir from the first script. You will also need to create a subdirectory off of that called 'VRT', which is where this script creates its new files. inputareas is a list of Ordnance Survey Grid Letters that the script will generate VRTs for. It will generate one set of files for each grid letter, you have to tell it which ones to do. After that, you can load the newly created VRT files into QGIS using the Add Raster Layer button and style them as per my original blog post.

Now back to those two different hillshades I mentioned. Why two different hillshades? If you imagine a hillshade a shining a light from a particular angle to create highlights and shadows, a linear feature running in the same direction as the direction of the light will not show up very well, so I've created a second hillshade with the light coming in at a 90 degree angle to the first hillshade. You can see the difference below.

First hillshade. Click for larger image.

Second hillshade. Click for larger image.

If you click on the images and look at the highlighted feature, a possible new (unconfirmed) Roman road on the Isle of Wight, you will see that it is much clearer in the second image compared to the first. My script generated the hillshades with light coming in from the north-east in the first image, which is parallel to the road feature, and from the north-west in the second image, which is perpendicular to the road feature, showing it up better.